Reversible electrostatic binding of charged solutes to an oppositely charged immobilized exchanger, followed by elution with a competing ion or a pH change, defines ion-exchange chromatography (IEX). Cation exchangers carry sulfonate or carboxylate groups; anion exchangers carry quaternary ammonium or diethylaminoethyl groups; both may be realized on resins, celluloses, silicas, or thin layers ^[1]. Retention is a function of the solute’s net charge, the exchanger’s capacity and pKa, and the ionic strength and pH of the buffer ^[2]. Modern IEX of biopharmaceuticals uses high-resolution particle and monolith formats to resolve charge variants of proteins, which remains the same electrostatic mechanism ^[3]. Conceptual boundaries exclude reversed-phase and hydrophilic-interaction chromatography, affinity chromatography, and electrophoretic separations that lack a chromatographic exchanger. Essential features are a defined ionogenic ligand, a buffered aqueous mobile phase, and a salt or pH program that displaces bound ions. Urea and related additives may modulate secondary (non-ionic) contacts without changing the primary ion-exchange definition ^[4].